The Sunyaev-Zel'dovich effect describes the interaction of hot electrons
in the dense intracluster medium of rich galaxy clusters with CMB photons.
Since the plasma temperature in rich cluster galaxies (6X107 K)
is much higher than that of the CMB (~3 K), approximately 1% of CMB photons
traversing a dense cluster of galaxies are inverse Compton scattered from
low to high frequency leaving a decrement in the amount of power seen on
the Rayleigh-Jeans side of the blackbody spectrum with a compensating increment
on the Wien side. This spectral distortion is independent of redshift,
providing a unique way to probe the largest cosmological scales.

The fact that the SZ effect is redshift independent can be exploited in
order to produce a distance-ladder independent measurement of the Hubble constant.
The CMB distortion for a given cluster depends on the density of hot intracluster
electrons integrated along the line of sight. The X-ray emission depends
on the density squared integrated along the line of sight. Therefore, assuming
equilibrium conditions and a spherical cluster, the comparison of SZ data
and X-ray observations provides a direct measurement of the cluster size
and therefore the distance. This distance estimate is used to constrain the
Hubble constant without making assumptions about cluster luminosity evolution.

Since clusters of galaxies collapse from large volumes of order 1000 Mpc
3, the ratio of baryons to dark matter should be a reasonable approximation
to the mix in the universe as a whole. Therefore, observations of the SZ
effect allow for the determination of the gas mass fraction in clusters.
Making the approximation that the cluster gas mass fraction reflects the
gas mass fraction of the universe as a whole, and taking constraints on the
baryon density imposed by nucleosynthesis, measurements of the SZE provide
an estimate of the matter density in the universe.

In addition, since the SZ effect is independent of redshift, it is possible
to produce a large survey of high redshift clusters selected only by their
masses (a very difficult thing to do using X-ray observations). The number
of expected such clusters as a function of redshift depends on the cosmological
model. Observations of SZ clusters at high redshift therefore can be used
to distinguish between cosmological models.